First Principles pK_a Calculations on Carboxylic Acids Using the SMD Solvation Model: Effect of Thermodynamic Cycle, Model Chemistry, and Explicit Solvent Molecules

Abstract

Aqueous pK(a) values are calculated from first principles for a set of carboxylic acids using the SMD solvation model with various model chemistries, thermodynamic cycles, and treatments of explicit solvation. In all, 108 unique theoretical protocols are examined. The direct (D) and water proton exchange (PX) cycles are trialled along with a new approach, termed the semidirect (SD) cycle. The SD thermodynamic cycle offers some improvements over the D and PX schemes, as it bypasses the gas-phase heterolytic bond dissociation calculation required in the conventional D approach while also avoiding an aqueous OH(-) calculation required by the PX method when using water as the reference acid. With all three cycles, the recommended model chemistry employs M05-2X/cc-pVTZ Gibbs energies of solvation with a single discrete water molecule and a high-level composite method for the gas-phase reaction energies. With the SD cycle, these calculations result in a mean unsigned error of less than 1 pK(a) units, with respective mean signed error and maximum unsigned error of less than 0.5 and 2 pK(a) units. Similar results are obtained with the D and PX cycles, and further improvement is required in both the gas and aqueous phase ab initio energy calculations before we can truly discriminate between the thermodynamic cycles investigated here.

Modeling the IR Spectra of Aqueous Metal Carboxylate Complexes: Correlation between Bonding Geometry and Stretching Mode Wavenumber Shifts

“…In Sutton et al, [14] it was shown that, using the M05-2X/cc-pVTZ level of theory with the SMD solvent model [15] for modeling the vibrational wavenumbers of free carboxylate anions in aqueous solution, am ean unsigned error of 4cm À 1 for n a and 28 cm À1 for n s was obtained. This model chemistry was originally proposed for modeling carboxylic acids in Sutton et al, [16] by determining the most accurate methods to calculate the pK a .T his model chemistry is utilized in this investigation. All calculations are performed using Gaussian 09.…”

Solvation and Protonation of Coumarin 102 in Aqueous Media: A Fluorescence Spectroscopic and Theoretical Study

“…For the description of the solvent effect in the condensed phase, both in the ground and excited states, the SMD 45 solvation model of Truhlar and co-workers were employed (in the following referred to as implicit model or Imp for short), which is based on the conductor polarized continuum model (CPCM). 46,47 According to previous studies, the PCM approaches satisfactorily describe the electrostatic effect of water molecules on solutes similar to C102 48 , while the SMD method is widely used for systems dominated by non-electrostatic interactions 49 and frequently applied to the calculation of ground-state pK a -s. 50 Two approaches of the CPCM solvation model were employed: 51 the equilibrium (eq) and the non-equilibrium (non-eq) solvation, which take into account if the solvent molecules have enough time to reorient according to the electron density of the solute. In addition to the implicit approaches the specific solvent interactions in the aqueous solution were also modeled by adding an explicit water molecule (implicit-explicit or For the simulation of the experimental absorption spectra vertical excitation energies and oscillator strengths were computed at the optimized ground-state geometries.…”

Theoretical calculations of the relative pKa values of some selected aromatic arsonic acids in water using density functional theory

“…Accordingly, many researchers are tending to calculation of pKa values using theoretical and computational approaches. 21,[24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] Recently quantum chemical calculations, using thermodynamical parameters such as Gibbs free energy, have reported good correlations between calculated and experimental acidities of a wide range of organic and inorganic acids. 18,[42][43][44][45][46][47][48][49][50] For example, gas phase and solution phase acidities of some organic compounds investigated by Namazian et al and Kheirjou et al reported good correlation between ab initio calculations of pKa values with the experimental data.…”